1. Field of the Invention
This invention relates to controlling mosquito metamorphosis. More particularly, it relates to the use of organic compounds which prevent or inhibit the growth of mosquito larvae into adult insects. This invention especially relates to the use of phenolic derivatives as mosquito development inhibitors.
2. Description of the Prior Art
There are a variety of techniques for controlling insect populations. In one common method, an insecticide, typically a synthetic organic compound, is applied to the insects or their habitat. This particular method often has undesirable side-effects since the insecticide may cause harm to humans, animals and useful insects, such as bees. Some organochlorine and organophosphorus compounds proved very effective and provided an initial high degree of safety to vertebrates but later proved potentially toxic to vertebrates because they attacked biological processes vertebrates shared with anthropods. Further, these compounds were often very stable, i.e., they were not biodegradable and thus tended to persist in the environment and in animal tissues. In addition, some insects developed a tolerance to these compounds to the point that the insecticides were of limited effectiveness. In view of these drawbacks, alternate means of insect control have been sought.
Effective control of insects has more recently been obtained by utilizing chemical compounds which act as anti-procreants. When one type of anti-procreant is administered, the insects become sexually sterile so that when mated with fertile insects, the eggs which are laid do not yield any progeny. Other anti-procreants produce the result that the female species does not lay any eggs and consequently no progeny result. These compounds are known, respectively, as chemosterilants and oviposition inhibitors.
Another approach to insect control, which is environmentally acceptable, exploits the hormones by which an insect regulates its growth and development. Two of the major insect hormones are the juvenile hormones (JH) and molting hormones (MH) or ecdysones. These hormones regulate insect growth and maturation. In the growing process, the insect larvae must molt--shed their rigid cuticles and replace them with new ones. Larval molting requires both JH and MH materials. Larvae die because of abnormal development when JH is present at the incorrect time. JH materials have been developed more readily than the ecdysones since the latter are steroids whose complexity often makes synthesis difficult and costly.
Application of JH insecticides is somewhat restricted because they are effective only at certain specific stages of insect life--hence timing is extremely important. A major problem in use of JH analogs is their ineffectiveness early in larval life. When the larval form is the pest, as for example caterpillars of moth species, they may cause considerable damage before they die. Thus, in the present state of the art, hormonal insecticides are most suited for use against light manifestations for which the amount of damage is economically tolerable, or against adult pests, such as mosquitos or flies. Compounds that inhibit the synthesis or action of these compounds so as to prevent molting at all stages of development, including the earliest would be most useful in this service.
These so-called "third generation pesticides" control insect population by inhibiting or preventing insect larvae or pupae from reaching the adult stage of insect development. Such compounds are referred to in the art as juvenile hormone (JH) mimics or insect development inhibitors. These agents do not kill the larvae, but rather prevent the growth thereof beyond the larval or pupal stage. Consequently, the number of adults is substantially reduced. The mimics actually cause several different situations, all of which result in controlling insect population. First of all, most of the treated larvae do not reach adulthood. Thus, the larvae survive for a period of time (possibly an entire growing season) as either larvae or pupae, and then die. During that period the larvae are, of course, very susceptible to predation and injurious climatic conditions. Furthermore, they are themselves incapable of reproduction, thus reducing the insect population for the next growing season. Secondly, some of the treated larvae may develop to various stages of adulthood. For example, the adult insect may only partially eclose, i.e., emerge from the larval or pupal shell. On the other hand, full eclosion may occur but the adult insect is either malformed or dead. In either case, the population of adult insects is substantially reduced.
The growth-inhibiting compounds have many advantages over insecticides and the like. First, the growth-inhibitors do not yield unwanted ecological side effects. Secondly, since the growth inhibitors act as juvenile hormone mimics, the insects do not develop a tolerance to the compounds. Thus, the compounds will not eventually become ineffective. Third, the growth inhibiting compounds are not harmful to beneficial insects or mammals because they are quite specific for a particular kind of insect.
It has been reported that some phenol derivatives have biological activity that mimics that of natural insect juvenile hormones and function as insect development inhibitors. Sacher reported that 2,6-di-t-butyl-4(alpha-alpha-dimethylbenzyl)phenol prevents mosquito larvae from metamorphosizing (31 Mosquito News (4) 513 (1971)). Schaefer et al. disclosed insect development inhibition for similar phenolic compounds wherein the substituent in the fourth position contained sulfur plus Co or CCl (Proc. 42nd Ann. Conf. Calif. Mosquito Control Assn. 147 (1974)). Schaefer and another co-worker found high JH mimic activity for several esters of dodecadienoic acid and an epoxy-phenoxyoctene (65 J. Econ. Ent. (4) 1066 (1972)).
The patent art also discloses specific JH mimics. U.S. Pat. No. 3,839,586 of Ludvik discloses that substituted phenols of the formula ##STR1## where R is hydrogen, isopropylphenyl or a C.sub.1 -C.sub.6 alkyl, alkoxy or thioalkyl and R.sub.1 is tertiary-butyl, tertiary-pentyl or cyclohexyl, prevent mosquito pupae from developing properly when applied to the larvae. These 2,6 substituted phenols include the compound evaluated by Sacher. Ludvik found, however, that not all 2,6 substituted phenols exhibit this inhibition. Thus 2,6di-isoamyl phenol, 2,6-di-t-butyl-4-chlorophenol and 2,6-di(methylnonyl)phenol show no such activity.
Other phenolic compounds similar to those of Ludvik were also found to exhibit mosquito development inhibition. U.S. Pat. No. 3,920,846 of Hanauye et al discloses a series of t-butyl-benzyl phenols while U.S. Pat. Nos. 3,973,040 and 4,082,814 of Jurd disclose a number of poly-t-butyl-cinnamyl phenols which function as JH mimics.
U.S. Pat. No. 3,920,844 of Barer et al (N-haloaminodiarylalkanes and N-haloaminodiarylhaloalkanes), U.S. Pat. No. 3,941,777 of Madsen et al (oximethers of certain aldehydes and ketones) and U.S. Pat. No. 3,941,842 of Metcalf et al (p,p'-disubstituted alphatrichloromethylbenzylanilines) all disclose specific organic compounds which exhibit JH mimic activity when applied to mosquito larvae thereby preventing proper development to the adult stage.
It is apparent from the prior art that some organic compounds, particularly substituted phenols, function as mosquito development inhibitors while some do not. It appears that this activity cannot be predicted but can only be determined from actual use.
It is an object of this invention to provide a method of inhibiting mosquito development utilizing organic compounds not employed previously for this purpose.
It is another object of this invention to provide a method of inhibiting mosquito development utilizing organic chemicals available commercially but not employed for this purpose heretofore.
It is a further object of this invention to provide a method of inhibiting mosquito development utilizing substituted phenols which have not been employed for this purpose heretofore.